The present invention relates to methods for electrical mobility spectrometry and, more particularly, to methods for multiple-charge correction of SEMS data.
Electrical mobility spectrometers and scanning electrical mobility spectrometers (“SEMS”) are commonly used for size distribution measurements of sub-micron particles. SEMS measurements require particles to be charged and this is often achieved by passing them through a bipolar diffusion charger, where the particles acquire an equilibrium charge distribution. The charged particles are then classified according to their electrical mobilities in a differential mobility analyzer (“DMA”) or any other mobility classifier. The number concentrations of the classified particles can be obtained using a particle detector, such as a condensation particle counter (“CPC”) or an electrometer.
To determine the size distribution of the sampled particles, the number concentration data of the mobility-classified particles must be inverted considering the instrument characteristics. If the DMA is operated under high resolution and the presence of multiply-charged particles can be ignored, the particle size distributions can be determined straightforwardly from the measured signal using a singly-charged, zeroth order inversion approach.
To account for the contribution of multiply-charged particles to the measured signal, a correction to the singly charged size distribution is required. This correction typically requires accurate knowledge of the largest particles sampled into the DMA, and a practical implementation of this requirement is achieved using an inertial impactor upstream of the aerosol sample flow that eliminates all particles beyond a critical size. Under ideal DMA and impactor operation, this approach will result in accurate size distribution measurements.
SEMS measurements are sometimes made under a range of non-ideal conditions, including: absence of an upstream impactor, non-ideal operation of the impactor, and/or operation under a partial voltage scan range. Under high-flow or low-pressure operation of a DMA, the pressure drop across the impactor may be problematic and under such conditions, the DMA may be operated without an impactor.
Even if an impactor is used upstream of the DMA, the discrepancy between the particle aerodynamic size that determines the impactor efficiency and the mobility size relevant to DMA measurements could complicate size distribution calculation from SEMS measurements. For example, SEMS measurements of size distributions of soot agglomerate or carbon nanotube particles were found to be affected by the difficulty of impacting out particles larger than the impactor cut-size. Even when the mobility and aerodynamic sizes are matched, the impactor is only effective if its cut-size matches the largest singly-charged particle size that can be classified by the DMA. When the DMA operating conditions may not permit scanning to the highest design voltage, such as when the DMA is operated on an aircraft where the upper limit of the voltage operation is pressure-dependent, the impactor cut-size may not match upper size limit of the DMA.
Accordingly, there is a continued need for multiple-charge correction of SEMS data.